Such an apparatus is known from international patent publication WO 94/02059. In that publication, the apparatus, as shown in FIG. 11, comprises clamping means which engage a circumferential edge of the cover for pressing the cover and the holder towards each other when the cover closes off the holder. A disadvantage of the known apparatus is that the frequent opening and closing of the holder reduces the reliability of the liquid seal between the cover and the holder. When, in use, under high pressure, hot water is supplied to the holder for preparing coffee, it may therefore happen that water leaks from the holder. This is caused by improper closure of the cover and the holder. If water leaks out, even if only to a slight extent, the pressure in the holder decreases. When the pressure in the holder decreases, this leads to a deterioration of the quality of the coffee extract obtained. This is the case in particular when the outflow opening has such a small diameter that the coffee extract spouts from the outflow opening for obtaining coffee with a small-bubble froth layer (cafe creme). When the pressure decreases, however, it may happen that the speed at which the coffee extract spouts from the outflow opening is not sufficient for preparing cafe creme.